In a drilled well, representative samples of rock are often cored from the formation using a hollow coring bit and transported to the surface for analysis. To collect these core samples, a number of coring methods may be used, including conventional coring and sidewall coring. With conventional coring, the drillstring is first removed from the wellbore and then a rotary coring bit with a hollow interior for receiving the cut core sample is run into the well on the end of the drillstring. Sidewall coring, on the other hand, involves removing the core sample from the bore wall of the drilled well. There are generally two types of sidewall coring tools, rotary and percussion. Rotary coring is performed by forcing an open, exposed end of a hollow cylindrical coring bit against the wall of the bore hole and rotating the coring bit against the formation. Percussion coring uses cup-shaped percussion coring bits, called barrels, that are propelled against the wall of the bore hole with sufficient force to cause the barrel to forcefully enter the rock wall such that a core sample is obtained within the open end of the barrel. The barrels are then pulled from the bore wall using connections, such as cables, wires, or cords, between the coring tool and the barrel as the coring tool is moved away from the lodged coring bit. The coring tool and attached barrels are finally returned to the surface where core samples are recovered from the barrels for analysis
In a typical percussion coring tool, an explosive device is used to propel the barrel from the tool into the surrounding formation. This explosive device is usually electrically fired, meaning an electrical current is used to initiate the explosion. Because these explosive devices are electrically initiated, they may be inadvertently initiated by stray voltage, static charge buildup, and radio frequency energy. In populated areas, sources of radio frequency may include CB radio, cellular telephones, radar, microwaves used for special communication and heat generation, conventional radio signals, power lines, high power amplifiers, high frequency electrical transformers, coaxial cables, etc. With respect to locations offshore, another source of radio frequency is powerful land-based transmitters used to communicate with equipment located on offshore platforms. Given the vast number of stray radio frequency sources, shutting these sources down temporarily so that sidewall percussion coring may be performed is impractical, if not impossible, particularly in congested areas near land-based oil and gas fields.